Field of the Invention
The present application relates to a laser driver, in particular, the present application relates to a circuit for driving a semiconductor laser diode (hereafter denoted as LD) accompanied with a function of the multi-level amplitude modulation (AM).
Related Background Art
The United States Patent Application published as US20140321864A has disclosed a technique to generate a multi-level signal by driving a modulator which is divided into multi-segments. Another United Stated Patent published as US20130027763A has disclosed a technique to generate a multi-level amplitude modulation signal by a digital-to-analog converter (D/A-C).
In a recent optical communication system, as the transmission speed of the communication system has been further accelerated, an advanced system using the pulse amplitude modulation (hereafter denoted as PAM) has been requested substituting for the modulation using the non-return-to-zero (NRZ) signal; because a conventional technique to modulate an LD directly and/or indirectly using an external modulator shows some limitations in an operation speed thereof. When an electrical PAM signal is converted into an optical PAM signal, the electrical PAM signal generated by a multi-bit D/A-C drives an LD under an assumption that the LD shows a linear dependence of the output optical signal on the input electrical signal. Generally, a linear amplifier sets an operating point thereof in a region where the linear relation between the output and the input is obtained, because a nonlinearity of a slope degrades the power efficiency and resultantly increases the power consumption of the amplifier.
A circuit disclosed in the US20130027763A is a driver generating a PAM signal by a D/A-C, where respective amplifiers are unnecessary to be operated linearly but a circuit size to generate the PAM signal becomes extremely larger because one binary signal is converted into a multi-level signal, for instance, a binary signal of three (3) bits is converted into a PAM signal with 8 levels. The circuit disclosed in the US20140321864A generates a PAM signal by dividing an optical device such as an LD and/or a modulator into several segments and modulates the respective segments by signals different from each other. The circuit disclosed therein may be reduced a size thereof compared with the circuit disclosed in the former reference, but the optical device and the driver circuit are necessary to be designed concurrently.
Accordingly, the optical device applicable to such circuit is limited to, for instance, a Mach-Zehnder modulator which is generally costly device.